The present invention relates to a wet image forming apparatus employed as, e.g., an electrophotographing apparatus or electrostatic recording apparatus, to form an image by using a liquid developer.
A wet image forming apparatus of this type has advantages that cannot be realized by a dry image forming apparatus, and its value has received attention in recent years. More specifically, since the wet image forming apparatus can use toner with very fine particles on the submicron order, it can realize high image quality. This apparatus is economical since a sufficiently high image density can be obtained with a small amount of toner, while achieving quality equivalent to that of a print.
A conventional wet image forming apparatus using liquid toner has several problems that allowed the dry type technique to dominate the market. One of the problems occurs in transferring a toner image.
The major problem in transfer is degradation in image quality. Conventionally, since a toner image attached to the image carrier is directly transferred by a transfer unit to a sheet with an electric field, transfer nonuniformity occurs due to variations in electric field corresponding to the unevenness on the surface of the sheet. Also, defective transfer tends to occur due to variations in electric characteristics of the sheet and environment dependency, thus greatly degrading the image quality of the transfer image.
In order to solve this problem, Jpn. Pat. Appln. KOKAI Publication No. 8-254907 discloses an apparatus in which a pressing force applied during transfer with an electric field from the image carrier to the sheet is changed in accordance with the type of the sheet or by changing a transfer nip width.
With this apparatus, even if unevenness on the sheet or the absorption for the solvent changes, a sufficiently large amount of carrier liquid is kept filled between the sheet and image carrier by adjusting the pressing force for the sheet, so that good transfer is enabled.
In this case, the sheet and the image carrier are, strictly speaking, out of mutual contact because the pressing force must be adjusted such that an appropriate amount of carrier liquid is present between the sheet and image carrier. Further, even if the sheet and the image carrier contact each other, the pressing force for the sheet and image carrier is 1 kgf or less in the whole transfer nip, which is equivalent to a pressure of less than 250 g/cm2.
This method has limitations and cannot assure a high image quality for all sheet types. More specifically, the surface roughness of plain paper and the like changes depending on surface portions, and the absorption for the solvent and the like are very unstable. Therefore, improvement in image quality is limited.
A large number of apparatuses have been proposed that do not transfer an image with an electric field directly from the image carrier to the sheet but transfer it from the image carrier to an intermediate transfer medium once and then to the sheet. U.S. Pat. Nos. 5,148,222, 5,166,734, 5,208,637, and the like disclose apparatuses that transfer a toner image from an image carrier to an intermediate transfer medium with an electric field (to be referred to as primary transfer hereinafter) and after that transfer the toner image from the intermediate transfer medium to a sheet with a pressure (and heat) (to be referred to as secondary transfer hereinafter).
Jpn. Pat. Appln. KOKOKU Publication No. 46-41679, Jpn. Pat. Appln. KOKAI Publication No. 62-280882, and the like disclose apparatuses that do not employ electric field transfer but employs pressure (and heat) in both primary transfer of the toner image from the image carrier to the intermediate transfer medium and secondary transfer of the toner image from the intermediate transfer medium to the sheet.
In these cases, secondary transfer of the toner image from the intermediate transfer body to the sheet is achieved by heat and pressure. An image of very high quality can therefore be obtained. However, a necessary transfer load largely changes, depending on the types of sheets, particularly the surface roughness of each sheet.
A load necessary for transfer can be small if the sheet is art paper with a smooth surface or special paper coated with a material having good adhesion with the toner image in advance. When, however, a toner image is to be printed on plain paper, tissue paper, or the like, a very large load is required.
Generally, the surface roughness of paper can be measured by the JIS-B0601 method. Measured by this method at ten point on the or an ordinary paper sheet, the average surface roughness (Rz) of the sheet ranges about 14 xcexcm to 30 xcexcm, and the center-line average roughness is about 2 to 4 xcexcm. Tissue paper has a larger surface roughness than the ordinary paper. By contrast, art paper has a ten-point average surface roughness (Rz) of 10 xcexcm or less in most cases, and its center-line average roughness (Ra) is 1.5 xcexcm or less. Like art paper, film such as an OHP sheet has a smaller surface roughness than the ordinary paper. If the sheet is art paper or the like on which the toner image can be transferred easily, the pressing contact force for the intermediate transfer body and sheet can be 1 kgf to 20 kgf (corresponding to a pressure of approximately 0.25 kg/cm2 to 5 kg/cm2), and if the sheet is plain paper or the like, a high pressure of 20 kgf to 200 kgf (corresponding to a pressure of 5 kg/cm2 to 50 kg/cm2) is required.
Conventionally, secondary transfer is performed with a high load regardless of the types of sheets, that is, whether the sheet is plain paper or art paper. Accordingly, a large load is continuously applied to the intermediate transfer medium to shorten its service life. A damage is also large when paper jamming or the like occurs.
This tendency is more typical in the transfer method that performs primary transfer of the toner image from the image carrier to the intermediate body with a pressure, because only a small degree of freedom is allowed for the design of the intermediate body and a highly durable intermediate transfer body is accordingly difficult to form.
The present invention has been made in view of the situations described above, and has its object to provide an image forming apparatus and image forming method in which the pressure is controlled so as to decrease the transfer load when a toner image is to be transferred to art paper or special paper which is processed in advance so that a toner image attaches to it easily, and so as to increase the transfer load when the toner image is to be transferred to a sheet with a poor surface smoothness such as plain paper, so that the intermediate transfer medium will not be damaged.
In secondary transfer of the toner image from the intermediate transfer medium to the sheet, the toner image is fused by applying heat. Hence, the longer the transfer time, the more advantageous for transfer.
It is, therefore, another object of the present invention to provide an image forming apparatus in which, when a toner image is to be transferred to a sheet such as plain paper with a poor surface smoothness, the transfer speed is decreased or the transfer nip width is widened, so that good transfer can be performed even if the type of the sheet changes.
An idea similar to the present invention is already disclosed in a fixing apparatus and the like in dry electrophotography. Generally, when the sheet is an OHP sheet, the toner must be dissolved sufficiently to render color. To cope with this, the fixing speed is often decreased to be lower than that for plain paper. Jpn. Pat. Appln. KOKAI Publication No. 7-325511 and the like exemplify this.
An example in which the transfer conditions are controlled by detecting the type of the sheet includes Jpn. Pat. Appln. KOKAI Publication No. 9-96942. In this example, if the sheet is an OHP sheet or the like, transfer is performed with transfer conditions different from those for plain paper.
These inventions are based on the premise that they deal with an OHP sheet, and they do not deal with the unevenness on the sheet or surface treatment. These inventions are not a solution considering the service life of the intermediate transfer body either, and have a purpose completely different from that of the present invention, as a matter of course.
In secondary transfer of the toner image from the intermediate transfer medium to the sheet with the pressure or heat and pressure employed in the present invention, even if a thin toner layer is to be transferred to a sheet with an uneven surface, the toner image cannot enter the recesses of the paper, and the lower the image density, the lower the transfer efficiency. This is the characteristic feature of wet electrophotography.
It is, therefore, still another object of the present invention to provide an image forming apparatus in which, when a toner image is to be printed on a sheet with a poor surface smoothness such as plain paper, the image density is intentionally increased so a low-density portion will not be formed in the toner image, thereby maintaining well the state of the image obtained by secondary transfer.
As an invention similar to the present invention, Jpn. Pat. Appln. KOKAI No. 8-254907 discloses an example in which the amount of toner attaching to the image carrier is controlled in accordance with the types of sheets.
This example relates to direct electric field transfer from the image carrier to the sheet, and aims at compensating for the transfer toner remained on the whole image, which cannot be sufficiently compensated for by adjusting the pressing force of the transfer unit, by changing the amount of toner attaching to the image carrier.
The present invention relates to pressure transfer and not electric field transfer. Exposure is controlled to be corrected so a low-density portion will not be formed in the toner layer. Therefore, the present invention is basically different from the disclosed examples.
Japanese Patent No. 2950414 and U.S. Pat. No. 6,002,891 discloses the technique of applying an appropriate transfer pressure, regardless of the thickness of the paper used, by detecting the thickness of the paper as the paper passes through the nip between guide rollers and by moving the backup roller up or down in accordance with the thickness detected. The technique has been invented to prevent application an undesirable pressure to the paper in the image-transferring process in the case where a thick paper sheet has passed the nip between the guide rollers. If such a pressure is applied to the paper, it will be applied via the intermediate transfer medium to the photosensitive image carrier, inevitably causing a trouble, such as deforming of the image, at the time of writing the latent image.
In the present invention, the thickness of the paper is not detected at all. Nor is the transfer pressure maintained at a constant value.
According to the present invention, there is provided an image forming apparatus comprising an image forming unit for forming an electrostatic latent image on an image carrier, a developing unit for developing the electrostatic latent image on the image carrier with a developing solution obtained by dispersing developer particles in a solvent, thereby forming a visible image, an intermediate transfer medium abutted against the image carrier to transfer the visible image on the image carrier thereto, a transfer unit having a backup member abutted against the intermediate transfer medium, the backup member urging a transfer target against the intermediate transfer medium, thereby transferring the visible image on the intermediate transfer medium to the transfer target, a determination unit for determining a type of the transfer target to which the visible image is to be transferred by the transfer unit, and a control unit for variably controlling a pressing force applied by the backup member to the transfer target in accordance with the type of the transfer target determined by the determination unit.
According to the present invention, there is also provided an image forming apparatus comprising an image forming unit for forming an electrostatic latent image on an image carrier, a developing unit for developing the electrostatic latent image on the image carrier with a developing solution obtained by dispersing developer particles in a solvent, thereby forming a visible image, an intermediate transfer medium to which the visible image on the image carrier is to be transferred, a transfer unit having a backup member abutted against the intermediate transfer medium, the backup member urging a transfer target against the intermediate transfer medium, thereby transferring the visible image on the intermediate transfer medium to the transfer target, a determination unit for determining a type of the transfer target to which the visible image is to be transferred by the transfer unit, and a control unit for variably controlling a contact time when the transfer target is in contact with the intermediate transfer medium in accordance with the type of the transfer target determined by the determination unit.
According to the present invention, there is also provided an image forming apparatus comprising an image forming unit for forming an electrostatic latent image on an image carrier, a developing unit for developing the electrostatic latent image on the image carrier with a developing solution obtained by dispersing developer particles in a solvent, thereby forming a visible image, an intermediate transfer medium to which the visible image on the image carrier is to be transferred, a transfer unit having a backup member abutted against the intermediate transfer medium, the backup member urging a transfer target against the intermediate transfer medium, thereby transferring the visible image on the intermediate transfer medium to the transfer target, a determination unit for determining a type of the transfer target to which the visible image is to be transferred by the transfer unit, and a control unit for controlling latent image forming operation of the image forming unit so as not to form a low-density image portion in the visible image in accordance with the type of the transfer target determined by the determination unit.
According to the present invention, there is also provided an image forming method comprising the steps of forming an electrostatic latent image on an image carrier, developing the electrostatic latent image on the image carrier with a developing solution obtained by dispersing developer particles in a solvent, thereby forming a visible image, transferring the visible image on the image carrier to an intermediate transfer medium abutted against the image carrier, urging a transfer target against the intermediate transfer medium with a backup member, thereby transferring the visible image on the intermediate transfer medium to the transfer target, determining a type of the transfer target to which the visible image is to be transferred, and variably controlling a pressing force applied by the backup member to the transfer target in accordance with the type of the transfer target determined by the determination unit.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.